Apple contractors regularly hear confidential medical information, drug deals, and recordings of couples having sex, as part of their job providing quality control, or “grading”, the company’s Siri voice assistant, the Guardian has learned.

Although Apple does not explicitly disclose it in its consumer-facing privacy documentation, a small proportion of Siri recordings are passed on to contractors working for the company around the world. They are tasked with grading the responses on a variety of factors, including whether the activation of the voice assistant was deliberate or accidental, whether the query was something Siri could be expected to help with and whether Siri’s response was appropriate.

Apple says the data “is used to help Siri and dictation … understand you better and recognise what you say”.

But the company does not explicitly state that that work is undertaken by humans who listen to the pseudonymised recordings.

Apple told the Guardian: “A small portion of Siri requests are analysed to improve Siri and dictation. User requests are not associated with the user’s Apple ID. Siri responses are analysed in secure facilities and all reviewers are under the obligation to adhere to Apple’s strict confidentiality requirements.” The company added that a very small random subset, less than 1% of daily Siri activations, are used for grading, and those used are typically only a few seconds long.

A whistleblower working for the firm, who asked to remain anonymous due to fears over their job, expressed concerns about this lack of disclosure, particularly given the frequency with which accidental activations pick up extremely sensitive personal information.

Siri can be accidentally activated when it mistakenly hears its “wake word”, the phrase “hey Siri”. Those mistakes can be understandable – a BBC interview about Syria was interrupted by the assistant last year – or less so. “The sound of a zip, Siri often hears as a trigger,” the contractor said. The service can also be activated in other ways. For instance, if an Apple Watch detects it has been raised and then hears speech, Siri is automatically activated.

The whistleblower said: “There have been countless instances of recordings featuring private discussions between doctors and patients, business deals, seemingly criminal dealings, sexual encounters and so on. These recordings are accompanied by user data showing location, contact details, and app data.”

That accompanying information may be used to verify whether a request was successfully dealt with. In its privacy documents, Apple says the Siri data “is not linked to other data that Apple may have from your use of other Apple services”. There is no specific name or identifier attached to a record and no individual recording can be easily linked to other recordings.

Accidental activations led to the receipt of the most sensitive data that was sent to Apple. Although Siri is included on most Apple devices, the contractor highlighted the Apple Watch and the company’s HomePod smart speaker as the most frequent sources of mistaken recordings. “The regularity of accidental triggers on the watch is incredibly high,” they said. “The watch can record some snippets that will be 30 seconds – not that long but you can gather a good idea of what’s going on.”

Sometimes, “you can definitely hear a doctor and patient, talking about the medical history of the patient. Or you’d hear someone, maybe with car engine background noise – you can’t say definitely, but it’s a drug deal … you can definitely hear it happening. And you’d hear, like, people engaging in sexual acts that are accidentally recorded on the pod or the watch.”

The contractor said staff were encouraged to report accidental activations “but only as a technical problem”, with no specific procedures to deal with sensitive recordings. “We’re encouraged to hit targets, and get through work as fast as possible. The only function for reporting what you’re listening to seems to be for technical problems. There’s nothing about reporting the content.”

As well as the discomfort they felt listening to such private information, the contractor said they were motivated to go public about their job because of their fears that such information could be misused. “There’s not much vetting of who works there, and the amount of data that we’re free to look through seems quite broad. It wouldn’t be difficult to identify the person that you’re listening to, especially with accidental triggers – addresses, names and so on.

“Apple is subcontracting out, there’s a high turnover. It’s not like people are being encouraged to have consideration for people’s privacy, or even consider it. If there were someone with nefarious intentions, it wouldn’t be hard to identify [people on the recordings].”

The contractor argued Apple should reveal to users this human oversight exists – and, specifically, stop publishing some of its jokier responses to Siri queries. Ask the personal assistant “are you always listening”, for instance, and it will respond with: “I only listen when you’re talking to me.”

That is patently false, the contractor said. They argued that accidental triggers are too regular for such a lighthearted response.

Apple differs from those companies in some ways, however. For one, Amazon and Google allow users to opt out of some uses of their recordings; Apple offers no similar choice short of disabling Siri entirely. According to Counterpoint Research, Apple has 35% of the smartwatch market, more than three times its nearest competitor Samsung, and more than its next six biggest competitors combined.

The company values its reputation for user privacy highly, regularly wielding it as a competitive advantage against Google and Amazon. In January, it bought a billboard at the Consumer Electronics Show in Las Vegas announcing that “what happens on your iPhone stays on your iPhone”.